H NMR Study of Hydrogenated Nanocrystalline Silicon Thin Films
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1153-A23-04
H NMR Study of Hydrogenated Nanocrystalline Silicon Thin Films
K.G. Kiriluk1, D. C. Bobela2, T. Su3, B. Yan3, J. Yang3, S. Guha3, A. Reyes4, P. Kuhns4, P.C. Taylor1 1
Colorado School of Mines, Golden, CO 80401 NREL, Golden, CO 80401 3 United Solar Ovonic LLC, Troy, MI 48084 4 National High Magnetic Field Lab, Tallahassee, FL 32310 2
ABSTRACT Hydrogenated nano-crystalline silicon (nc-Si:H) is a promising material for multijunction solar cells. We investigated the local hydrogen environments in nano-crystalline silicon thin films by nuclear-magnetic-resonance (NMR). At room temperature, 1H NMR spectra have broader components than those observed in standard device grade hydrogenated amorphous silicon (a-Si:H). As the temperature decreases, the 1H NMR exhibits a broadening of the line shape attributed to hydrogen atoms at the interfaces between the amorphous silicon (a-Si) and the crystalline silicon (c-Si) regions. These results suggest that the local hydrogen structure in nc-Si:H is very different from that in a-Si:H. In particular, the hydrogen clusters contributing to broadened spectra may exist on the surfaces of the a-Si/c-Si interfaces which do not exist in the more dense matrix of a-Si:H and may contribute to certain unique optoelectronic properties of these nc-Si:H thin films. The dependence of the spin-lattice relaxation time (T1) on temperature, however, is very similar to that in a-Si:H, which indicates the spin-lattice relaxation mechanism, i.e via spin diffusion through molecular hydrogen, is common to both systems. INTRODUCTION Advances in solar cell technology lies in nanotechnology. The most wellunderstood of these technologies is hydrogenated nanocrystalline silicon (nc-Si:H). ncSi:H absorbs photons at a lower energy than hydrogenated amorphous silicon (a-Si:H), which is why it can be used as the bottom cell absorption layer in multi-junction solar cells. nc-Si:H also has the advantage that it degrades less over time than a-Si:H [1]. During the growth process for nc-Si:H, silane gas is diluted with hydrogen to promote the growth of nanocrystals inside an amorphous matrix. The dilution ratio is varied to maintain constant density and size of the nanocrystals. Isolated 1H atoms passivate dangling bonds in the amorphous region, while others are believed to cluster together at the interfaces between the amorphous region and the crystallites. We have used 1H NMR to study the relative hydrogen content at different sites. To gain insight into the molecular hydrogen content of these systems, we have also investigated the temperature dependence of the spin-lattice relaxation in a nc-Si:H sample.
EXPERIMENT 1
H NMR experiments were conducted on a-Si:H and nc-Si:H samples deposited at United Solar Ovonics, LLC using very high frequency (VHF) plasma enhanced chemical vapor deposition (PECVD). A sample of thickness 1µm was deposited on aluminum foil. The foil was etched using a diluted 3% HCL solution. The remaining sample, now a powder, was then collected in an NMR tube and dried in ambient air.
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